Solar thermal energy increasing in popularity

Francis Killorin

Saturday

Apr 26, 2008 at 12:01 AMApr 26, 2008 at 5:41 PM

Efficiency is an important element in any energy application. With renewable energy, efficiency still is important, even though the fuel is free. Here, return on investment takes center stage, as in the case of solar thermal energy.

Efficiency is an important element in any energy application.

For example, if we burn natural gas in a boiler at home, since heat is what we want, and heat is the primary product of combustion, 91 percent of the BTUs in the incoming gas can be distributed through the rooms.

On the other hand, when gas is burned to generate electric energy, the power plant wants to use the violent expansion that occurs when a liquid changes to a gas. That mechanical energy uses about 30 percent of the energy stored in the gas. The remaining 70 percent is just heat, and in this application, there is no use for it. The efficiency of this application is only 30 percent, and the resultant electricity contains only about one-third as much energy as the gas that was burned.

With renewable energy, efficiency is equally important, even though the fuel is free. Here, return on investment takes center stage, as in the case of solar thermal energy.

On a clear day, the sun delivers about 300 BTUs per square foot to anything on the earth’s surface that is facing it directly. Note that this makes square footage of exposure a very important factor in the use of the sun. Also, the term “right angle” is important, because the sun, that’s angle over the earth is constantly changing, is rarely right over any particular point.

Subject to these limitations, solar thermal can be an efficient energy source.

The fuel is free; it is easy to collect without high-tech equipment; and it can be stored in floors and interior walls, or in water.

Solar heat is used in several ways.

First, there is passive solar, very helpful in heating space in the temperate zones. It is the easiest of all, but it has to be designed into the building before it is built.

The design elements are simple:

- Use 6-inch exterior walls, to save the heat stored by the inner walls.

- Face the long dimension of the house toward the sun, to let the most sun in during the heating season.

- Extend the southern overhang to at least 24 inches, to keep the sun out during the summer. Opaque white shades at each end will also help.

Since the passive solar investment is very low, the return on investment is very high.

The solar water heater is also a good solar thermal application. In fact, hot wash water is needed everywhere for comfort and health.

In the developed countries, it is considered a necessity. In less affluent areas, millions need it but do not have it, and if they must buy fossil fuel to get it, they never will.

The residential solar water heater is low-tech, starting with two or three of the flat-plate collectors that have been visible on roofs since the ’70s.

Because collection occurs in daylight but usage usually occurs early in the morning or late in the day, storage is essential. The storage tank can be anything from an insulated rain barrel to a 120-gallon tank of inexpensive, durable polyethylene, holding temperatures of up to 160 degrees.

To move the collected heat into storage, the stored water can be pumped through the collectors to pick up the heat collected there.

In the temperate zone, temperatures will drop below freezing at night, so the water must drain back into the storage tank when the sun is no longer heating the collectors.

Finally, most building codes require that the water that goes through the collectors cannot be used in the faucets. So, the household water to be heated goes through a heat exchanger, a simple copper coil, immersed in the solar-heated water, on its way to the faucets and washing machines.

Any stored heat that does not transfer into the wash water this time simply waits in the tank for the next demand.

This method is called drain-back and is very efficient.

A handy homeowner can install a system like this for less than $3,000. With tax credits and rising fuel costs, it could pay for itself in four years.

E-mail franergy@verizon.net.

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